The present article is devoted to the issue of unity of laws, patterns and mechanisms of evolution at all its stages and levels of Big History and megaevolution. Despite the enormous differences between cosmic, planetological, chemical, biological, and social evolutions, there are many similarities. There are a lot of important and insightful works on the development of complexity in our Universe and in the course of Big History. Unfortunately, much less studies are devoted to analysis of universal similarities, patterns and rules within Big History. Mostly such research is focused on a few laws of Big History which are usually connected with development of complexity. However, laws in terms of the typological similarity of many patterns and rules in star-galaxy, planetological, chemical, biological and social phases of Big History, are of great importance. In the present article we will consider a number of such important similarities, which, in our opinion, clearly demonstrate the systemic-structural and functional-evolutionary unity of the world at its different levels and in different areas. The understanding of these similarities deepens our perception about all stages of Big History and its regularities, and leads us away from the false idea that social evolution in all aspects is different from the evolution of previous levels. In the first section our key goal is to give our own definitions of evolution which would cover as many variants of evolutionary changes as possible. In the second section we will try to give a rather voluminous and dialectical picture of the unfolding universal evolution instead of a short scheme: cosmic – biological – social. The notions of main and transitional phases of Big History are introduced; and the importance of its planetary and chemical phases is shown. In the third section we will show that one can reveal a number of similarities at all levels and phases of megaevolution, which can be generalized in universal laws, rules, mechanisms, patterns and principles of evolution. One should note that in fact none of the important laws and principles, not any of the important rules of evolution, have been ‘lost’ in the process of transition from lower to higher levels. They were only modified and became more complicated, and there also appeared some new principles and rules (and in retrospect one can see their rudiments at the lowest levels of evolution). Some of these laws and rules are described in this section. In the fourth section we will try to present some evolutionary and philosophical ideas that explain the profound similarity in the laws and patterns of megaevolution at all its levels and phases. In the conclusion we will discuss evolutionary and non-evolutionary matters.